With regard to hard-held and powered tools used to drive features into or out of an item, especially those used in medical applications, there are several common problems associated with tools incorporating existing torque-limiting devices. These problems include loss of consistent torque value after repeated autoclave sterilization cycles, internal components breaking due to high forces and loads on internal cams and gears, inconsistent torque values due to wear on internal components, a strong recoil or snap when set at higher torque values, and difficulty in servicing the mechanism.
More particularly, as shown in FIGS. 20 and 21, in prior art torque-limiting devices, the devices include gears 100, 101 including a number of generally angular teeth 102 disposed along one side of the gears 100, 101. Each tooth 102 includes an angled sliding surface 104 and a flat, vertical locking surface 106 located between the sliding surfaces 104 of adjacent teeth 102. These gears 100, 101 are positioned in the mechanism with the teeth 102 facing one another in a manner where one of the gears 100 can rotate with respect to the other gear 101. This is due to the construction of the mechanism in which one gear 100 is fixed to mechanism and the other gear 101 can move with a drive body (not shown) for the tool to provide the torque-limiting function. When the tool incorporating the gears 100, 101 is subjected to a torquing force greater than a preset maximum, the moveable gear 101 rotates with respect to the fixed gear 100, such that the sliding surfaces 104 of the opposed teeth 102 slide against one another and urge the fixed gear 100 against a spring member (not shown) that biases the gears 100, 101 towards one another. The movable gear 101 can continue to rotate in response to the excessive torque until the flat locking surface 106 on the opposed teeth 102 are moved past the edges 105 of the sliding surfaces 104. In this position the gears 100, 101 move or snap back towards one another due to the bias of the spring member, and the respective flat surfaces 106 come into contact with one another to secure the gears 100, 101 in a camming position.
In order to enable the prior art mechanism to provide a closely controllable amount of torque resistance, the mechanism requires that the forces biasing the gears 100, 101 towards one another from: 1) the spring member; 2) the surface friction provided by the contact of the angled surfaces 104 on the opposed teeth 102 sliding with respect to one another; and 3) the drag of the gears 100, 101 on a housing (not shown) for the mechanism all be known and properly maintained. To enable the surface friction and drag to be controlled, a proper amount of lubrication is required to be present both on the teeth 102 and on the back of the rotatable gear 101 in contact with the housing in order to maintain the constant drag forces on the angled surfaces 104 and the movable gear 101. However, due to the cleaning and/or sterilization of tools including devices of this type, each sterilization cycle causes an inherent loss of the lubrication in the mechanism. As a result, the amount of surface friction and drag between the gears 100, 101 changes over time. This in turn drives the torque values up such that a consistent amount of torque resistance is not provided by the device.
Further, as a result of the particular shape of the teeth 102 on each gear 100, 101 the rotation of the gear 101 results in the locking surfaces 106 on each gears 100, 101 “snapping” into engagement with one another in both the axial and circumferential directions after passing one another. This movement of the locking surfaces 106 into engagement with one another necessarily creates vibrations in the mechanism which are transmitted through the mechanism and the tool incorporating the mechanism to the fastener and/or the person on which the device is being utilized. In many situations, these vibrations are highly undesirable. Also, the stress exerted on the surfaces 106 as they strike one another also leads to fracturing or chipping of the teeth 102, lessening the useful life of the mechanism. When the teeth 102 are chipped, this additional material can also collect on the sliding surfaces 104 of the teeth 102, thereby causing even more inconsistent torque values for the mechanism.
In addition, prior art torque limiting devices include one piece calibration nuts (not shown) that engage the spring members of the mechanism to calibrate or set the amount of torque necessary to rotate the gears 100, 101 with respect to one another. The calibration nut is normally secured to the mechanism by adhesives, by pairs of jam or locking nuts to reduce space and/or a mechanical interruption of threads to which the calibration nut is mounted. The design of each of these prior art calibration nut assemblies increases the complexity of the overall mechanism, and provides an additional manner in which the mechanism can break down.
Due to the multitude of problems associated with prior art torque limiting devices, it is desirable to develop or design a torque-limiting device which greatly reduces each of the problems associated with prior art devices at this time.